As a suggestion, do something that other carts don't do, such as having dual ROM and dual scam, with a physical switch to change which is used, if you added an RTC to this you could sell a cart that would be able to play both Red and Blue on one cart, or Gold and Silver, or Tetris and Qix.
Honestly make a product that stands apart from the rest, and then you'll have the room to make money and such.
Also make sure it's a completed product, or at least in a physical beta version before attempting to sell it